Porosity control in piezoelectric films

ABSTRACT

A piezoelectric film having a porosity between 20 and 40%, a thickness ranging from tens of microns to less than a few millimeters can be used to form an ultrasonic transducer UT for operation in elevated temperature ranges, that emit pulses having a high bandwidth. Such piezoelectric films exhibit greater flexibility allowing for conformation of the UT to a surface, and obviate the need for couplings or backings. Furthermore, a method of fabricating an UT having these advantages as well as better bonding between the piezoelectric film and electrodes involves controlling porosity within the piezoelectric film.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.14/240,168, filed Feb. 21, 2014, which is a national phase entry ofInternational Patent Application No. PCT/CA2011/000955, filed Aug. 24,2011, the entire content of which is herein incorporated by reference.

FIELD OF THE INVENTION

The present invention relates in general to a technique for forming apiezoelectric film with controlled porosity, especially for thefabrication of ultrasonic transducers as may be utilized fornondestructive testing (NDT), structural health monitoring (SHM), orbiomedical diagnostics.

BACKGROUND OF THE INVENTION

There is substantial demand for piezoelectric ultrasonic transducers(UTs), such as those formed by piezoelectric films, sandwiched betweentwo electrode layers. In a wide variety of contexts, the ability topropagate ultrasonic waves in a medium, and/or detect waves thuspropagated, is highly useful, for example, in ultrasonic non-destructivetesting (NDT) and structural health monitoring (SHM) of materials,components or structures. In some applications, there is a particularneed: to use broad frequency bandwidth UTs; to perform ultrasonicgeneration or detection at elevated temperatures; or to conform thepiezoelectric transducer to components or structures that have complexshapes, such as curved surfaces like pipes. For example, there is a needto monitor the thickness of pipes in a power plant that is subjected tohigh temperatures, wear, corrosion or erosion. If a high accuracy of thethickness measure is desired, a broad bandwidth UT is advantageous.Typically it is highly desirable that UTs operate in the broadbandfrequency regime in which their −6 dB bandwidth exceeds 30%, toemit/detect ultrasonic pulses that have only a few ring-down cycles,allowing for high precision thickness measurement and also the defectlocation, if any. Therefore there are high demands for UTs that canperform ultrasonic measurements efficiently and accurately (a) on curvedsurfaces, (b) at high temperatures with capability to sustain thermalcycles from low temperature such as −80° C. to elevated temperaturessuch as 200° C., 500° C., 800° C. or 1000° C.; and (c) a broad frequencybandwidth. It is also desirable to have flexible UTs which have theabove (a), (b) and (c) features.

While broad frequency bandwidth can be provided by mechanically dampingthe UT, e.g. with a backing between a top electrode of the UT and itsair interface (i.e. on the opposite side of the inspected surface).Backings attenuate the ultrasonic (mechanical) energy, as is known inthe art (see, for example U.S. Pat. No. 3,376,438 to Colbert [1], U.S.Pat. No. 3,989,965 to Smith et al. [2], and G. Kossoff, “The effects ofbacking and matching on the performance of piezoelectric ceramictransducers”, IEEE Trans. on Sonics and Ultrasonics, vol. SU-13, pp.20-30, March 1966. [3]). When the piezoelectric element is excited bythe electrical signal, the generated ultrasonic wave transmitted intothe backing material will be attenuated by the mechanisms of absorptionand/or scattering. It suppresses multiple reflected echoes inside thepiezoelectric element and thus it will have the broadband frequencycharacteristics. However, the use of backings in accordance with theprior art, introduce other problems. Backing materials often use epoxiesas the host materials in which metal or ceramic powders are filled inorder to increase the acoustic impedance of the backing and match withthat of the UTs. This backing makes the previous arts bulky, heavy, andnot flexible and difficult to be used to evaluate materials, componentsand structures with complex surfaces. In addition, epoxies cannotsustain a temperature more than several hundred degrees Celsius so thatit is not suitable for high temperature applications. It should also benoted that for measurements at high temperatures, thermal cycles mayhappen. Because of the large difference in thermal expansioncoefficients and thermal conductivity between the epoxy and the thinmetallic electrode of the UT, epoxy-based backings tend to detach fromthe electrode after several thermal cycles. Such detachment causes anabrupt failure that degrades the broadband frequency characteristics ofthe UTs.

Another approach to achieving broad frequency bandwidth UTs is to inserta matching layer between the UT and inspected surface, the matchinglayer having the proper acoustic impedance according to those of thepiezoelectric transducer material and the medium, as known in the art(see, for example [3], U.S. Pat. No. 2,430,013 to Hansell [4], or U.S.Pat. No. 4,016,530 to Goll [5]). When one layer with quarter wavelengththickness and proper acoustic impedance, i.e. square root of the productof the acoustic impedance of piezoelectric material and that of thetarget to be monitored, are inserted between the UT and the medium, thebandwidth can be increased. However, it is relatively difficult toobtain such a proper acoustic impedance material. Also, the increase ofbandwidth using this technique is limited. Multiple layers could be usedto accomplish the acoustic impedance conditions, although it can causefurther loss and the design becomes more complicated. Furthermore,acoustic impedance will change when the temperature changes, and eachmaterial has a different acoustic impedance dependence with temperature.Therefore, it is at least difficult to provide high quality impedancematching with a variety of ultrasonic media, for operation across a widetemperature range, which is often required for NDT and SHM applications.

According to the teachings of U.S. Pat. No. 4,751,013 to Kaarmann et al.[6], porosity is introduced into piezoelectric films with a view toreducing shear wave excitation at the transducer edge and to match theacoustic impedance of the UT to that of the substrate to be inspected,so that more ultrasonic energy can be transmitted from the UT to thesubstrate. There is no information relating porosity to frequencybandwidth, or temperature of operation, and no evidence that bondingduring thermal cycle or flexibility would be provided. Furthermore, thedisclosed method of fabricating porous piezoelectric films was by mixingpiezoelectric powder, binder, and polymer in the form of small particleswhich were fired out during calcination process. Since the sizes ofpearl polymers were between 10 μm to 40 μm which are large, highultrasonic attenuation and strong scattering at high ultrasonicoperation frequency are expected.

U.S. Pat. No. 6,111,339 to Ohya et al. [7] teaches manufacture of porouspiezoelectric sheets. There is no information relating the porosity tothe frequency bandwidth, or operating temperature, and no evidence thatbonding during thermal cycle or flexibility would be provided.Furthermore, the disclosed method of fabricating porous piezoelectricfilms was by mixing piezoelectric powder, binder, and combustible powdersuch as poly methyl methacrylate which will burn out during heatingprocess. The pore sizes in this method were between 5 and 25 μm whichare still large and result in high ultrasonic attenuation due to strongscattering at high ultrasonic operation frequency.

U.S. Pat. No. 5,585,136 to Sekimori et al. [8] teaches a particularfabrication technology, sol-gel technique, to produce piezoelectricfilms for ultrasonic transducers. The invention reported is related tohow to reduce the porosity in order to fabricate dense piezoelectricfilms. There is no information relating the porosity to the frequencybandwidth, or temperature of operation, and no evidence that bondingduring thermal cycle or flexibility would be provided. Also US patentapplication US2008/182128 to Boy et al. [9] teaches a method to producelow porosity piezoelectric films with high piezoelectric constant bymultiple impregnation of the porous film with sol-gel piezoelectricprecursor solution. This method is laborious and results inpiezoelectric transducers that are narrow band and do not have the hightemperature capabilities.

It is also known to provide high porosity UTs. For example, U.S. Pat.No. 5,958,815 to Loebmann et al. teaches a method of producing aparticular piezoelectric film for a transducer designed for coupling toa gaseous medium. As noted in the field of that invention, the notablydifferent acoustic impedance of solids and gasses make conventionalultrasonic sensors and actuators made of dense ceramic andceramic-polymer composites, of limited use in coupling to gaseous media.Loebmann therefore only advocates use of porous UTs for coupling tocoupling with gaseous media. The prior art shows a bias for dense UTswhen coupling with solids or liquids. It will be noted that their UTsare about 80% porous, making them ill suited for coupling to solid orliquid media.

Accordingly there is a need for broad frequency bandwidth ultrasonictransducers capable of operating at high temperatures such as 200° C.,500° C., 800° C. or 1000° C., preferably without requiring a backing.

SUMMARY OF THE INVENTION

Applicant has discovered, unexpectedly, that porosity, to a controlleddegree, is an important feature for designing UTs for operation inspecific temperature ranges, and for emitting pulses having highbandwidth. Furthermore, higher porosity piezoelectric films exhibitgreater flexibility allowing for conformation of the UT to a surface.The UTs may be mounted without any coupling or backing, which isadvantageous in many applications, may be provided for operation atelevated temperatures, or within a particular range of elevatedtemperatures, and may exhibit better bonding between the piezoelectricfilm and electrodes than non-porous, or otherwise fabricatedpiezoelectric films.

In accordance with the present invention an ultrasonic transducer (UT)is provided. The UT comprising a piezoelectric film sandwiched betweentwo electrodes, wherein the film is 2 microns to 2 mm thick, has aporosity of 15-40% with micron-scale or sub-micronscale pores, and isprincipally composed of piezoelectric powders having micron or submicronsizes mixed with a residue of a binder. The thickness may be 10 micronsto 1 mm, or 50 microns to 1 mm. The porosity may be 22-40%, 25-40%,30-38%, 30-35%, or 22-32%. The binder residue may include 1 residue of aliquid or gel oxidizing agent that formed an intermediate oxidationlayer on at least one of the electrodes, 2 a residue deposited afterthermal treatment that is piezoelectric or 3 a residue deposited afterthermal treatment that is chemically and thermally stable at a desiredoperating temperature of the UT, that has a high dielectric constant,preferably higher than the powders. The binder residue may include twoor more of these 3, and may include substantially nothing other thanthese 3. The film may consist of the binder residue and powders.

The electrode may be a high electrical conductivity material withminimal and non-fragile oxidation at temperatures throughout a desiredoperating temperature of the UT.

The UT may have a −6 dB bandwidth greater than 30%, 60%, 70%, or 100%,such as a range of 70-200%, 100-150%, or 93-133%.

One of the electrodes may be directly coupled to a surface of a part ofan apparatus for emitting or detecting ultrasonic waves in the part,without an impedance matching layer, or a backing. The UT may bedesigned for high-temperature applications.

Also, in accordance with the present invention a method of producing anultrasonic transducer (UT) with controlled porosity is provided. Themethod comprises providing a bottom electrode for the UT; mixing abinder and piezoelectric powders to form a slurry; depositing theslurry, and drying, sintering the deposited slurry to build up apiezoelectric film on the bottom electrode; poling the piezoelectricfilm to make it piezoelectricly active; applying a top electrode for theUT; and providing an electric circuit for controlling the piezoelectricfilm. A size distribution, shape distribution and porosity of thepowders, and an abundance of the binder relative to that of thepiezoelectric powders in the slurry as deposited, are controlled toprovide a desired porosity for the piezoelectric film that is between15-40%, 20-40%, 25-40%, 30-38%, 30-35%, or 22-32%.

The bottom electrode may be a high electrical conductivity material withminimal and non-fragile oxidation at temperatures throughout a desiredoperating temperature of the UT, and may be bonded to a surface of apart to be tested ultrasonically. The binder may be selected to leavethe residue described above.

In accordance with the method, the powders may constitute about 40% toabout 90% molar ratio of the mixture, with the balance being a binder(ignoring the entrained air). The binder is preferably a ceramicprecursor, such as a liquid or sol-gel. For example, the mixture mayinclude a molar ratio of around 80:20-40:60 powder to precursor,80:20-60:40, or 75:25-70:30. Mixing may involve limited comminuting ofthe powders, to provide sufficient porosity to the film, for example bylimiting the amount, duration or degree of ball milling, or byultrasonic excitation of the slurry to provide sufficient mixing,without substantial comminution. The slurry may be deposited by screenprinting, stencil printing, spray coating, tape casting, dip coating orspin coating the slurry onto the electrode, and may preferably beapplied by spray coating, as with controlled spray velocity distributionand distance. A number of coats may be applied to provide a layerthickness prior to drying, sintering and poling, the layer thicknesscontrolled to provide sufficient drying to impart a desired porosity tothe resulting film.

Further features of the invention will be described or will becomeapparent in the course of the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the invention may be more clearly understood, embodimentsthereof will now be described in detail by way of example, withreference to the accompanying drawings, in which:

FIG. 1 is a schematic illustration of a thin substrate UT in accordancewith an embodiment of the invention;

FIG. 2 is a schematic illustration of a thin substrate UT array inaccordance with an embodiment of the invention;

FIG. 3 is a schematic illustration of a thick substrate UT in accordancewith an embodiment of the invention;

FIG. 4 is a schematic illustration of a thick substrate UT array inaccordance with an embodiment of the invention;

FIG. 5 is a microscope image of a top surface of a piezoelectric film inaccordance with an example of the invention; and

FIG. 6 is a microscope image of a cross-section of a piezoelectric filmin accordance with an example of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention provides piezoelectric films having improved hightemperature operation, and bandwidth, provided by porosity control, andteaches how to fabricate such piezoelectric films. The preferredapplications of the invented piezoelectric films are for ultrasonictransducers (UT) for NDT, SHM, and biomedical diagnostics. The thicknessof such piezoelectric films may range from several microns to less thantwo millimeters. The porosity of the piezoelectric film may becontrolled between about 15% and about 40%. The UTs may be designed tooperate in a broad ultrasonic bandwidth, at temperature of up to 1000°C., or may be flexible when such piezoelectric films are directly coatedonto thin membranes made of metals or polymer composites. Herein a broadultrasonic bandwidth refers to a −6 dB bandwidth of more than 30% of thecenter operation frequency. Such flexible UTs can conform to curvedsurfaces such as pipes.

Thick porous piezoelectric film UTs consisting of a top electrode, aporous piezoelectric film and a bottom electrode, on substrate arepreferred. The porous piezoelectric films are typically made of ceramicssuch lead-zirconate-titanate (PZT), bismuth titanate, lithium niobate(LiNbO₃), etc. The average size of the pores is of microns orsub-microns.

To fabricate the UT, a bottom electrode is deposited onto a substrate.Where desired, the substrate may be flexible. The bottom electrode maybe composed of metals or alloys suitable for high temperature operation,having high electrical conductivity, with minimal and non-fragileoxidation at the desired operating temperatures. For temperatures up to850° C., electrodes such as nickel, platinum, titanium, stainless steel,silver, etc. may be used. Both metals and polymer composites arepreferred, provided they can resist temperatures of the heat treatment(typically above 300° C.), and the desired operating temperature range.Fabrication temperature could be lowered down to 150° C. with signalstrength and chemical stability sacrifice. The metal substrates can benickel, platinum, titanium, stainless steel, silver, etc., while polymercomposites can be glass fiber composites, carbon fiber composites,polyimide based composites, etc. The bottom electrode can be formed onthe thin substrate by electroplating or electroless plating, spraycoating, painting, vacuum deposition, etc. The bottom electrode canalternatively be the substrate.

A mixture is prepared with piezoelectric film materials in powder form,having micron or submicron sizes, with oxidizing binders in a liquid orgel form. The composition of the piezoelectric powders is preferablychosen for high piezoelectricity at the desired operating temperature,which may be at a high operating temperature. The mixture may bedeposited onto the bottom electrode, by screen printing, stencilprinting, spray coating, tape casting, dip coating, and spin coating,for example, to produce a layer of the mixture.

The layer is heat treated, during which treatment the materials aredried and calcined, some portions of the binder evaporate and react withthe materials, resulting in a porous piezoelectric film. The depositionof layers and drying may alternate, or may be in series, depending onthe duration and desired degree of the drying. The binder residue, afterthe heat treatment, preferably has a high dielectric constant,preferably higher than that of the piezoelectric powders. Such highdielectric constant is crucial for the electrical poling of the porouspiezoelectric film together with the bottom electrode. After thecalcining, the film is subjected to a high DC voltage, which provideselectrical energy to pole the material, aligning dipoles of thepiezoelectric materials, making the material piezoelectricly active.During the electrical poling, an electric field extends across both thepiezoelectric powders and the binder material, and so it is importantthat the binder residue does not conduct electricity, as this wouldinterfere with the poling.

Preferably the binders create an oxidation layer with the bottomelectrode during the heat treatment, resulting in strong adhesionbetween the porous film and bottom electrode. In order to strengthen thepiezoelectricity of the film, the binder material after the heattreatment and electrical poling, is preferred to be a piezoelectricmaterial that can work at the desired temperatures, such as up to 200°C., 500° C., 800° C. or 1000° C. Suitable binders include piezoelectricceramic precursors of a wide variety of recipes known in the art, eachhaving different limitations and advantages. Some examples are providedin the following papers, the contents of which are incorporated hereinby reference: PZT sol-gel precursors such as D. Barrow, C. V. R. V.Kumar, R. Pascual and M. Sayer, “Crystallization of sol gel PZT onaluminum and platinum metallizations”, Mat. Res. Soc. Symp. Proc., vol.243, pp. 113-122, 1981, N. Tohge, S. Takahashi and T. Minami,“Preparation of PbZrO₃—PbTiO₃ ferroelectric films by the sol-gelprocess”, J. Am. Ceram. Soc., vol. 74, no. 1, pp. 67-71, 1991, and T.Olding, B. Leclerc, M. Sayer, “Processing of multilayer PZT coatings fordevice purposes”, Integrated Ferroelectrics, vol. 26, pp. 225-241, 1999;and bismuth titanate sol-gel precursors such as X. S. Wang, Y. J. Zhang,L. Y. Zhang, X. Yao, “Structural and dielectric properties of Bi₄Ti₃O₁₂thin films prepared by metalorganic solution deposition”, Appl. Phys. A,vol. 68, pp. 547-552, 1999, P. Fuierer and B. Li, “Nonepitaxialorientation in sol-gel bismuth titanate films”, J. Am. Ceram. Sic., 85[2], pp. 299-304, 2002, and M. Toyoda, Y. Hamaji, K. Tomono, and D. A.Payne, “Synthesis and characterization of Bi₄Ti₃O₁₂ thin films bysol-gel processing”, Jpn. J. Appl. Phys., vol. 32, pp. 4158-4162,September 1993.

The top electrode layer is then deposited. The top electrode layer mayhave similar requirements, but may not need to suffer exposure to theheat treatment step, as it may be deposited after heat treatment.Alternatively the top electrode may be deposited prior to heat treatmentand poling, whereby the same oxidation layer is created between thepiezoelectric film and both electrodes.

The porosity is necessary to achieve the broad bandwidth, the highoperation temperature and flexibility of the UTs. The control of theporosity and the average sizes of the pores can be achieved by adjustingthe sizes of the piezoelectric powders, the mixing ratios of thepiezoelectric powders with respect to binders, compositions of thebinders, deposition (such as spray coating parameters: coating velocityand thickness), and heat treatment parameters. Principally, the size ofthe powders, and mixing conditions of the precursor, thickness of thelayer of mixture, and the weight ratio of powder to precursor, have beenfound to reliably control the porosity of the resulting film in someapplications. It is conventional to ball mill the mixture prior tospraying, as this has the effect of comminuating the powders, densifyingthe mixture, and making the mixture more homogeneous. By ultrasonicmixing instead of ball milling, the powder is not comminuated, leavinglarger pores. These larger pores are filled with the precursor solution.By limiting the amount of precursor solution, therefore, it is possibleto further increase porosity of the film.

Applicant has observed that in general, the lower the porosity, thenarrower the UT's frequency bandwidth (ceteris paribus). A piezoelectricporous film having porosity less than about 10% typically needs to havea backing to achieve broad bandwidth emission/detection. Backingmaterials are taught in the prior art references [1-3]. A piezoelectricporous film having porosity higher than 40% will typically haveinsufficient piezoelectricity for coupling to solid or liquid media, andwill typically exhibit high ultrasonic scattering losses at higherultrasonic frequencies.

Applicant has experimented with a variety of techniques for forming alead-zirconate-titanate (PZT), and bismuth-titanate powder-based UTs.Specifically the techniques used are similar to those taught in thepapers listed above, and involve producing a precursor solution, andadding a powder to the mixture, spraying the mixture, calcining(sintering) and poling, and applying electrodes. The specific precursoris not believed to be essential to the result, but how to best achievethe desired porosity can vary somewhat between formulations. There arenumerous recipes for precursors, and different recipes produce UTshaving different power, thickness, efficiency, durability, operatingtemperature, and cost. In general the first parameters to consider forproducing a desired bandwidth, flexibility and thermal operation(including thermal cycling resistance), are powder size, shape, porosityand distribution, as well as the ratio of the powders to precursor inthe mixture, and thickness of the layer. Other factors such asdeposition (spray) parameters, age of the precursor, thermal treatmentparameters, and nature of the powder and precursor all come into play,and may be varied. It is within the scope of the person of ordinaryskill to adapt known fabrication processes to produce the desiredporosity of the UTs.

For example, a series of UTs were produced with the PZT piezoelectricmaterial. The specific precursor is a sol gel containing titaniumbutoxide, zirconium butoxide, and lead acetate trihydrate. With theprecursor solution thus synthesized, PZT powder (200 mesh size) and theprecursor were mixed. As is conventional, multiple layers were appliedonto a metal substrate by spray coating. Before thermal treatment, 4 or8 coats were sprayed (by hand) to obtain a coating having homogeneousthickness. Films were created with each of five layers dried and firedat 120° C. and 650° C. for 5 minutes each. The film was poled withcorona discharge at 120° C. A 25 kV potential difference was used togenerate the corona discharge. After the poling, polishing was executedin order to have uniform thickness. Silver top electrodes were paintedonto thin porous PZT films at room temperature.

According to the first UT, the mixing of the PZT powder (40 wt. %) andprecursor (60 wt. %) was performed in a ball mill. The ball milling wasperformed for 2 days, i.e. long enough for saturation of size reduction,using balls of Burundum (0.5″ OD, 0.5″ height). It is estimated that theball milling reduced the powder size from about 10-20 μm (median ˜12 μm)prior to milling, to about 0.5-2 μm after milling. According to thesecond UT, the powder to precursor ratio was 33:67 wt. % to allow formore precursor to occupy the greater voids between the larger particles,as the powders were mixed in an ultrasonic bath and not ball milled.Except for a first layer, which had 4 coats, 8 coats were applied perlayer prior to thermal treatment, but otherwise the method was the same.According to the third UT, the powder to precursor ratio that of thefirst UT, and the mixing and layering were applied as per the second UT.

The porosity of the piezoelectric film in the first UT was determined tobe 22% by SEM observation. As a UT, it showed signal strength, that iscomparable to commercial ultrasonic transducers. The bandwidth wascalculated from the centre frequency and upper/lower −6 dB frequencies,and value obtained was 94%. The calculated velocity, derived from thefirst peak frequency and the film thickness, was 977 m/s. The second UThad a film porosity of 27% by SEM observation, showed a signal strength16 dB lower than that of the first UT, with a −6 dB bandwidth of 122%,and a velocity of 888 m/s. The third UT had a film porosity of 32%, asignal strength 26 dB lower than that of the first UT, a −6 dB bandwidthof 130%, and a 648 m/s ultrasonic velocity. Given the porosities, theUTs will have better flexibility, higher thermal operating ranges,better resistance to thermal cycling than dense UTs that are usuallypreferred because of their higher signal strengths.

The porosity of the piezoelectric film enables the UT to exhibit threeadvantages: (a) broad frequency bandwidth emission/detection, (b) highoperating temperature and resistance to thermal cycling, and (c)flexibility. The porosity of the piezoelectric film, which is coateddirectly onto the bottom electrode with high electrical conductivity,allows a large thermal expansion coefficient difference between thepiezoelectric porous ceramic film and the bottom electrode, withoutincreased risk of delamination. It also means that such porosity enablesthe porous piezoelectric transducers to operate at high temperaturesincluding thermal cycle conditions. Flexibility of the UTs may also bedesired. The flexibility of a dense piezoelectric thick film (i.e. 0%porosity and thickness >10 μm) is poor. The porosity of the thickpiezoelectric porous film together with the thinness of the topelectrode, bottom electrode, and thin substrate enable that the porousUT can be adapted to curved surfaces such as pipes.

FIG. 1 is a schematic illustration of a UT in accordance with anembodiment of the present invention. It will be appreciated thatprotective layers and other coatings may be added to this structure, aswell as circuitry for regulating current between the top and bottomelectrodes. The embodiment shown is of a UT, consisting of a topelectrode 1, a porous piezoelectric film 2, and a bottom electrode 3 ona thin substrate 4. The total thickness of the porous piezoelectric filmis less than two millimeters. The porous piezoelectric film is formedfrom piezoelectric ceramic powders having an average size in the micronor submicron range, and the size may be visible from electron microscopeimaging after the heat treatment. The thin substrate 4 can be composedof one or more metals, or polymer composites. The thickness should below enough to ensure the flexibility of the UT. The UT, as shown in FIG.1, can be conformed to a pipe.

Another embodiment of the invention is shown in FIG. 2, in whichmultiple top electrodes 5 of the porous UT are provided, to form a UTarray. The array can be in the form of circular or square dots, parallelstraight lines, partial and full cylindrical and circular lines withseparation distances between the adjacent dots or lines, for example.Each top electrode represents the active area of one UT. The array canbe operated as multiple individual UTs or a phase array which canprovide electronic scanning and focusing capability. Otherwise, thisembodiment is similar to that shown in FIG. 1.

Another embodiment of the invention is shown in FIG. 3, in which porousUTs are directly deposited on the thick substrate 6. As will beunderstood by those of skill in the art, a layer is thick if it has athickness of more than one ultrasonic wavelength. The thick substrate 6can be composed of metals or polymer composites with complex shapes suchas pipes.

Another embodiment of the invention is shown in FIG. 4, in whichmultiple top electrodes 5 of the porous UTs can be in arrayconfigurations on a substrate 6 that is more than one ultrasonicwavelength thick. In the drawings, like reference numerals refer to likefeatures, and the descriptions of the features are not repeated for eachdrawing.

Other advantages that are inherent to the structure are obvious to oneskilled in the art. The embodiments are described herein illustrativelyand are not meant to limit the scope of the invention as claimed.Variations of the foregoing embodiments will be evident to a person ofordinary skill and are intended by the inventor to be encompassed by thefollowing claims.

1. An ultrasonic transducer (UT) comprising a piezoelectric filmsandwiched between two electrodes, wherein the film: is 2 microns to 2mm thick, has a controlled porosity of 15-40% with micron-scale orsub-micronscale pores; and is principally composed of piezoelectricpowders having micron or submicron sizes mixed with a residue of abinder, so that the UT is endowed with a broad ultrasonic bandwidth ofat least 30%.
 2. The UT of claim 1 wherein the binder residue includes:residue of a liquid or gel oxidizing agent that formed an intermediateoxidation layer on at least one of the electrodes, which is a highelectrical conductivity material with minimal and non-fragile oxidationat temperatures throughout a desired operating temperature of the UT; aresidue deposited after thermal treatment that is piezoelectric; or aresidue deposited after thermal treatment that is chemically andthermally stable at a desired operating temperature of the UT, andhaving a high dielectric constant, preferably higher than that of thepowders.
 3. The UT of claim 1 wherein the binder residue comprises twoor more of: residue of a liquid or gel oxidizing agent that formed anintermediate oxidation layer on at least one of the electrodes, which isa high electrical conductivity material with minimal and non-fragileoxidation at temperatures throughout a desired operating temperature ofthe UT; a residue deposited after thermal treatment that ispiezoelectric; and a residue deposited after thermal treatment that ischemically and thermally stable at a desired operating temperature ofthe UT, and having a high dielectric constant, preferably higher thanthat of the powders.
 4. The UT of claim 1 wherein the binder residueconsists essentially of nothing other than: residue of a liquid or geloxidizing agent that formed an intermediate oxidation layer on at leastone of the electrodes, which is a high electrical conductivity materialwith minimal and non-fragile oxidation at temperatures throughout adesired operating temperature of the UT; a residue deposited afterthermal treatment that is piezoelectric; and a residue deposited afterthermal treatment that is chemically and thermally stable at a desiredoperating temperature of the UT, and having a higher dielectric constantthan the powders.
 5. The UT of claim 1 wherein the film consists of thepowders and the binder residue.
 6. The UT of claim 1 wherein the −6 dBbandwidth of the UT is greater than 30%.
 7. A high-temperatureultrasonic transducer (UT) comprising a piezoelectric film sandwichedbetween two electrodes, control circuitry for the film, wherein thefilm: is 2 microns to 2 mm thick, has a controlled porosity of 15-40%with micron-scale or sub-micronscale pores; and is substantiallycomposed of piezoelectric powders having micron or submicron sizes mixedwith a residue of a binder, and wherein one of the electrodes isdirectly coupled to a surface of a part of an apparatus for emitting ordetecting ultrasonic waves in the part at a surface opposite the film,without an impedance matching layer, and the UT does not include abacking.
 8. The high-temperature UT of claim 7 wherein the binderresidue includes: residue of a liquid or gel oxidizing agent that formedan intermediate oxidation layer on at least one of the electrodes, whichis a high electrical conductivity material with minimal and non-fragileoxidation at temperatures throughout a desired operating temperature ofthe UT; a residue deposited after thermal treatment that ispiezoelectric; or a residue deposited after thermal treatment that ischemically and thermally stable at a desired operating temperature ofthe UT, and having a high dielectric constant, preferably higher thanthat of the powders.
 9. The high-temperature UT of claim 7 wherein thebinder residue comprises two or more of: residue of a liquid or geloxidizing agent that formed an intermediate oxidation layer on at leastone of the electrodes, which is a high electrical conductivity materialwith minimal and non-fragile oxidation at temperatures throughout adesired operating temperature of the UT; a residue deposited afterthermal treatment that is piezoelectric; and a residue deposited afterthermal treatment that is chemically and thermally stable at a desiredoperating temperature of the UT, and having a high dielectric constant,preferably higher than that of the powders.
 10. The high-temperature UTof claim 7 wherein the binder residue consists essentially of nothingother than: residue of a liquid or gel oxidizing agent that formed anintermediate oxidation layer on at least one of the electrodes, which isa high electrical conductivity material with minimal and non-fragileoxidation at temperatures throughout a desired operating temperature ofthe UT; a residue deposited after thermal treatment that ispiezoelectric; and a residue deposited after thermal treatment that ischemically and thermally stable at a desired operating temperature ofthe UT, and having a high dielectric constant, preferably higher thanthat of the powders.
 11. The high-temperature UT of claim 7 wherein thefilm consists of the powders and the binder residue.
 12. Thehigh-temperature UT of claim 7 wherein the −6 dB bandwidth of the UT isgreater than 30%.
 13. A method of producing an ultrasonic transducer(UT) with controlled porosity, the method comprising: providing a bottomelectrode for the UT; mixing a binder and piezoelectric powders to forma slurry; depositing the slurry, and drying, sintering the depositedslurry to build up a piezoelectric film on the bottom electrode; polingthe piezoelectric film to make it piezoelectrically active; applying atop electrode for the UT; and providing an electric circuit forcontrolling the piezoelectric film, wherein a size distribution, shapedistribution and porosity of the powders, and an abundance of the binderrelative to that of the powders in the slurry as deposited, arecontrolled to provide a desired porosity for the piezoelectric film thatis between 15-40%, and a thickness of 2 microns to 2 mm.
 14. The methodof claim 13 wherein: the bottom electrode is a high electricalconductivity material with minimal and non-fragile oxidation attemperatures throughout a desired operating temperature of the UT; thebottom electrode is bonded to a surface of a part to be testedultrasonically; the binder is selected to leave a residue of a liquid orgel oxidizing agent that forms an intermediate oxidation layer on thebottom electrode; the binder is selected to leave a residue that ispiezoelectric; the binder is selected to leave a residue that ischemically and thermally stable at a desired operating temperature ofthe UT, and has a high dielectric constant, preferably higher dielectricconstant than the powders; the binder is selected to leave substantiallyonly residues that are: piezoelectric, or stable at a desired operatingtemperature of the UT with a high dielectric constant, preferably higherdielectric constant than the powders; mixing comprises mixing about 40to about 90% molar ratio of the powders with the binder; mixing involveslimited comminuting of the powders to provide sufficient porosity to thefilm; mixing involves ultrasonic excitation of the slurry to providesufficient mixing, without substantial comminution; depositing theslurry comprises screen printing, stencil printing, spray coating, tapecasting, dip coating or spin coating the slurry onto the electrode;depositing the slurry comprises spray coating the electrode; depositingthe slurry comprises spray coating the electrode with controlled sprayvelocity distribution and distance; or depositing the slurry comprisesspray coating the electrode in a number of coats to provide a layerthickness prior to drying, sintering and poling, the layer thicknesscontrolled to provide sufficient drying to impart a desired porosity.